Surface acoustic wave devices are small-sized and stable to temperature and to variations with time, can be given the desired filter characteristics by varying the configuration of comblike electrodes and therefore find wide application, for example, to IF filters for television receivers, IF filters for satellite broadcasing, RF converter osillators for VTRs, etc. It is desired that surface acoustic wave devices be as great as possible in effective electromechanical coupling coefficient K.sup.2 which represents the efficiency of conversion of electrical energy to surface acoustic wave energy.
We have already proposed a surface acoustic wave device which comprises a piezoelectric aluminum nitride film having a relatively large thickness (for example, of 20 .mu.m to 30 .mu.m) and formed on a substrate and in which the direction of C-axis orientation of the piezoelectric film is inclined with respect to a normal to the film (U.S. Pat. No. 4,868,444). The proposed device can be given a higher coupling coefficient than formerly possible.
If the substrate for forming the piezoelectric film thereon is a single crystal silicon substrate, other circuit devices can also be formed on the same substrate, whereby an electronic circuit including the surface acoustic wave device can be fabricated in the form of a single chip, hence an advantage. Further when the piezoelectric aluminum nitride film formed is thin, it becomes possible to produce the surface acoustic wave device with an improved efficiency than those wherein the piezoelectric film is of the bulk type or has a large thickness.
However, in the case of surface acoustic wave devices comprising a single crystal silicon substrate and a piezoelectric thin film formed thereon and having a thickness, for example of about 1 to about several micrometers, the coupling coefficient is dependent not only on the crystal structure, thickness and like characteristics of the piezoelectric thin film but also on the crystal structure and the like of the substrate, so that full research has yet to be made on improvements in coupling coefficient. In Unexamined Japanese Patent Publications SHO 58-59616, SHO 58-59617 and SHO 58-59618, for example, investigations were directed merely to the optimization of the thickness of piezoelectric aluminum nitride thin film, with the C-axis of the thin film set parallel or perpendicular to the single crystal silicon substrate. In this case, the coupling coefficient K.sup.2 was up to 1.03% even under optimum conditions.